Scala: “Parameter type in structural refinement may not refer to an abstract type defined outside that refinement”

Question

I\'m having a problem with scala generics. While the first function I defined here seems to be perfectly ok, the compiler complains about the second definition with:

Answer 1

Thanks, that really helped. Your answer pointed me into the right direction. I wrote the thing to this:

trait Lifter[C[_]] {
  class Wrapper[A](c: C[A])  {
    def >>=[B](f: A => C[B])(implicit m: Monad[C]): C[B] = { 
      m >>= (c, f)
    }   
    def >>[B](b: C[B])(implicit m: Monad[C]): C[B] = { 
      m >> (c, b)
    }   
  }

  implicit def liftToMonad[A](c: C[A]): Wrapper[A] = new Wrapper(c)
}

Thanks a lot.

Regards, raichoo

Answer 2

Filling in the blanks in your example, I made this compile:

trait Monad[C[_]] {
  def >>=[A, B](f: A => C[B]): C[B]
  def >>[B](a: C[B]): C[B]
}

trait Lifter[C[_]] {
  class D {
    def >>=[A, B](f: A => C[B])(implicit m: Monad[C]): C[B] = {
      m >>= f
    }
    def >>[B](a: C[B])(implicit m: Monad[C]): C[B] = {
      m >> a
    }
  }

  implicit def liftToMonad[A](c: C[A]) = new D
}